1. Field of the Invention
The invention relates to novel aluminum salts of alkylhydroxymethylphosphinic acids, their preparation and their use as flame retardants.
2. Description of the Prior Art
Polymers are frequently made flame retardant by adding to them phosphorus-containing or halogen-containing compounds or mixtures thereof. Some polymers are processed at high temperatures, e.g. at 250.degree. C. or above. For this reason, many known flame retardants are not suitable for such applications, because they are too volatile or are not sufficiently heat-stable.
Alkali metal salts of dialkylphosphinic acids are thermally stable and are already proposed as flame retardant additives for polyester (DE-A1-2 252 258). They must be introduced in amounts of up to 30% by weight and some have an adverse corrosion-promoting effect on the processing machinery.
Furthermore, the salts of dialkylphosphinic acids with an alkali metal or a metal from the second or third main group or subgroup of the Periodic Table of the Elements have been used for the preparation of flame-resistant polyamide molding compounds, in particular the zinc salts (DE-A1-2 447 727). Low-flammability thermoplastics may also be prepared by using said salts of phosphonic acids in combination with nitrogen bases such as melamine, dicyanodiamide or guanidine (DE-A1-28 27 867). A further large class of salts of phosphonic acids are the polymeric metal phosphinates. These are nonionic coordination complexes and are soluble in organic solvents. They are suitable as flame retardant components for halogenated aromatic polymers and for polyesters (U.S. Pat. No. 4,078,016; U.S. Pat. No. 4,180,495), polyamides (U.S. Pat. No. 4,208,321) and polyesters/polyamides (U.S. Pat. No. 4,208,322). The generally difficult industrial preparation of these metal phosphinate polymers is a disadvantage.
Dialkylphosphinic acids are prepared by free-radical catalyzed addition of olefins to monoesters of phosphonous acid and the subsequent hydrolysis of the dialkylphosphinic esters thus produced. The monoesters of phosphonous acid are produced by esterifying the phosphonous acids. The processes are technically complex and proceed via a plurality of stages. Technically simple preparation processes for salts of phosphonic acids are therefore sought.